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What is Crew Resource management (CRM)?
I hear a lot about CRM these days (it seems to be a buzz word). It is related to safety, but what exactly is it?
From what I know, it also applies to single pilot flights (even in a Cessna 150!), but where is the "crew" that is being managed in this case and how does it improve safety?

A good reading http://www.nasa.gov/offices/oce/appel/ask/issues/42/42i_crew_resource_management_prt.htm

CRM is about making use of all available resources to safely conduct a flight.  Pilots these days (even single pilot ops) have a wealth of resources available to them.  Anything you can see and anyone you can talk to is a resource, and CRM is about making efficient use of those resources.
Flying a light single you will have a subset of these resources:

Cockpit displays
Charts
Checklists
ATC
FSS
Flight Watch (EFAS)
Live Weather downlinks (e.g. XM satellite weather)

On a large transport aircraft these change a bit:

Cockpit displays
Charts
Checklists
ATC
FSS
EFAS
Onboard RADAR
The QRH (quick reference handbook)
The FOM (flight operations manual)
The other pilot(s)
The cabin crew
Passengers (Doctor in a medical emergency?  People helping in an evac?)
Dispatch
Medlink
Company Ops
For ground ops:

Fuellers
Ramp personell
Gate agents

More and more emphasis is placed on managing these resources as you move up the chain.  By the time you are taking your first 121 upgrade checkride, it is more about judgement and CRM than it is about the flying.

CRM is not just crew anymore - it's now typically referred to as "Cockpit Resource Management" (or in some cases, when no crew is present, as "Single-Pilot Resource Management") and it's something the FAA emphasizes on all checkrides.
CRM includes all resources available to any pilot. In a typical light GA aircraft this means checklists, instruments, gauges, radios, and nav. However, ATC is a resource, especially during abnormal or emergency situations. So is FSS, Unicom, or even other aircraft nearby (think relaying an IFR cancellation etc).
In a large aircraft, CRM obviously includes your flight and cabin crew, plus other airline or corporate perks like dispatch.
The safety improvements come from knowing when to offload work or call on systems or people for assistance. Even a non-pilot passenger can be a huge help in spotting other aircraft, tuning radios, rummaging around for your spare pen... that's all CRM and lets the pilot focus on flying.

From the Private Pilot PTS (FAA-S-8081-14B):

Special Emphasis Areas
Examiners shall place special emphasis upon areas of aircraft operations considered critical to flight safety. Among these are:
 15. Single-Pilot Resource Management (SRM), and
 16. Other areas deemed appropriate to any phase of the
practical test.

something the FAA emphasizes on all checkrides on my PP check ride things were going very well. The DPE had me deviate to an unplanned alternate that I had to locate on a chart. I got points for CRM by asking him, a certificated pilot, to fly while I read the chart... but he also (not surprisingly) told me no :)

@mah Nice! I had a couple of students try that with the same results, but examiners always love that kind of thing. It shows solid problem-solving skills.

@mah Good response. Until the examiner tells you to treat him like he doesn't exist, he's a resource. That's the point of CRM.

In Europe:
As far as I know, Crew Resource Management or Cockpit Resource Management training is only needed for commercial/airline flights where multi-pilot aircrafts are flown.
It's basically Multi Crew Co-operation (MCC) training (plus the communication aspect) with the purpose of increasing the efficiency of communication, coordination, decision-making and leadership in the cockpit.
In the end it breaks down to efficient pilot communication, efficient distribution of cockpit tasks and "inter-pilot-double-checks" (meaning a pilot checking the other pilot actions and vice-versa).

If that is true, why does the FAA private pilot PTS list it as an emphasis area? See: Single-Pilot Resource Management

Sorry guys, i forgot you're mostly Americans...
Americans are  usually faster legislating things, specially safety related, and sometimes overdoing it as well...
In Europe, CRM is only for Airliners and it's their responsibility to establish and maintain such training which is usually integrated with Line Oriented Flight Training...
Furthermore, I don't see much need for such training to fly a Cessna 150 visually, for God sake...
Although for single-pilot Very Light Jets, for instance, it's a different story.

If we were to call it common sense, or situational awareness, would you have a problem with it being used by private pilots?

No, definitely not. Such training is included in a PPL... And I, as a private pilot always try use all resources (either knowledge or devices) to keep my situational awareness as high as possible, especially in private VFR flights.


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What are the typical facets of ownership costs, for a single engine aircraft?
It's easy to go online and look at prices of a Cessna 172, but what are some examples of how to break down the real world costs of ownership?

How much other maintenance should you plan for?
How much does an engine overhaul cost?
Insurance
Hangar
etc..

It would be great to also get some typical costs and ranges, since some element are more predictable than others.  Obviously the costs will vary based on individual aircraft and location, as well as over time, but I'm looking for information that would help someone make the buy/rent decision.
I'm asking for the structure of the costs to start making a plan for cost analysis and diligence. For example, with an IFR aircraft, what costs are involved with keeping it current?

To anyone who voted to close or down voted, please comment about why. I think this is a common question among pilots, and it seems relevant, but if there is an objection to the question, I'd like to know what that objection is.

See also: Downvotes and closing votes without comments

I think this one is way too broad.  I'm familiar with these costs and the variation is absolutely huge.  I can't see any valid results coming from this.

I think this could be a good question when narrowed to a specific geography, but the variance is going to be huge. Just think about the cost of apron vs hangar parking - now how will that vary between {small regional airport} and {larger hub}

Ok, I disagree, but your comment is fair enough. What if the question was narrower to ask for a list of ownership costs? In other words, what costs are there, without regard for how much those costs are?

@kevin42 that seems like a reasonable modification to the question. Answers could provide examples of amounts, but it'd be important to note that they're not necessarily representative, due to geography / age of aircraft / condition of aircraft / etc.

While asking for an actual estimate is too broad. I believe asking for a list of possible costs and for information about where to go to get quotes or to find out if a particular cost would apply to you would be appropriate.

Great discussion of the cost of ownership of a Cessna 150 called "The affordable airplane": http://realcostofownership.blogspot.com/2013/08/the-affordable-plane.html

I've heard it said that the break-even point for owning vs renting is 200 hours/year. I fly a lot less than that, but I don't like sharing my toys, so I own anyway.
I make it a point to not actually count my cost of ownership, because it wouldn't be fun any more, but I'll give you some rough numbers.
I bought a 1962 Mooney for \$30,000. That's about as cheap as it gets, and this was reflected in the condition of the plane and the high-time engine.
In the first year had it, I spent \$18,000 on an engine rebuild, new prop, new carb, new magnetos, new oil pump, and all the other firewall-forward upgrades that were required when I got the new engine. Five years later, I spent \$14,000 on new paint. It still needs a new interior, but that can wait. I spent about \$3000 at West Texas Aero installing speed upgrades.
Budget \$10/hour for your next engine rebuild.
Annual inspection typically runs \$3000 to \$6000 depending on what they find wrong. I'm getting a tank resealed this year, so it's going to be an expensive one.
Parking runs typically \$50-\$100/month, depending on where you are.
Insurance for my old crate runs \$750/year. Expect to pay more for a nicer plane or if you have less experience. Note: get a commercial license; it will pay for itself in reduced insurance in the long run.
I think gas will typically run \$5-6/gallon, depending on a lot of factors. My Mooney burns around 9.5gph, and flies 142 kts so it's really economical in that respect (and the main reason I don't upgrade).
Light twins are cheap to buy, but that's because their cost of ownership is through the roof. A low-time pilot can expect to pay \$12,000/year for insurance and the annual inspection will also be a back-breaker.
I used to have money. Now I have an airplane.

I used to have money. Now I have an airplane. hahaha, I love it!

Or more precisely: you had money, now you have Mooney.

The rule of thumb I was always told was "If you can afford the buy 3 airplanes, you can afford to own 1."

Wow, that annual cost. Rough! Is that pretty standard for Mooneys?

Retracts and constant-speed props add a lot to the annual inspection cost, so it's going to be more than for say a C-172, but overall, I'd say I'm getting off cheap compared to a lot of other airplane owners.

I have been looking into this recently and have some numbers on hand from people I have spoken to,
I now fly out of KDYL but was perviously flying out KPNE, I always asked around for costs at both and this is what I found out. KDYL is a typical small strip in suburban PA, KPNE is a big airport in the northeast corner of Philadelphia (its not Philly international)
Hanger: I met a guy at KPNE who co-rented a hanger with 2 other guys from the major FBO there. They were paying 2800 a month (split 3 ways, so just about 940 each). The hanger fit 3 GA planes but was made for a larger regional jet. A tie down at KPNE was a but north of $150 a month from what I heard. Up at KDYL there is a 9 year waiting list for a Hanger (if you can even ever get one) so the thought of renting one there is out. I have yet to get a price on them since they are so hard to come by. Tie downs seem to be under 100 a month there.
Insurance: This is highly based on you and your time in type and time over all. Its hard to ballpark
Annual: Most of the Cherokee/172 people I talk to (fixed gear simple stuff) say their annual can be as low or lower than 1500 if nothing is wrong, up to 3000 if a few issues are found and the sky is the limit after that. I have been told by many people at KDYL to find an A&P that will allow owner assisted annuals (you open up and close the plane) which greatly reduces the cost. I have also heard that folding gear can almost double the annual cost but this seems mainly to be when comparing planes like an Arrow and Archer (almost the same plane with the exception of the gear).
My buddy has a beautiful '95 Saratoga HP he likes to say that any time anything on the plane breaks its a thousand bucks no matter what it is...
Engine: Most people in the 172/Cherokee world seem to put aside about 10-15 for engine maintenance but more importantly most of these planes actually make it to published TBO numbers. You can read up on it here but how much you set aside depends on when you buy the plane. Think about it like this, the idea is to set aside enough for a full overhaul when the engine needs it so a nice equation is
(TBO time - Time On Engine at Purchase)/(Cost of Overhaul) = Hourly Engine Set Aside
Other Maintenance: Some people set aside a prop reserve much like an engine reserve. Other things like breaks, spark plugs, tires wear out predictably and may need replacing. You should always have an emergency reserve for when that strange thing breaks that you didn't anticipate.
IFR: there is a nice checklist here and the full regs here but the general aircraft IFR requirements are

24 Calendar Months

Static pressure system
Altimeter
Encoder
Transponder

30 Days
VOR check

Airborne +/- 6 degrees
Ground +/- 4 degrees
Dual VORs w/i 4 degrees of each other

28 Days

GPS database

Impossible answer...but I'll try.  I owned a 1976 V35B and it cost between 15k-17k per year to own and fly 50-80hrs/yr.  This range included some non-standard repair items and if I REALLY tried I could have been sub-14k but that would have required some serious owner-assisted maintenance that I just didn't have time for.
Hangar rent can range between 100 and 1000 per month.  Outdoor parking can range from 0 to 200.  It all depends on where you live.
Insurance is the big hurdle for many new pilots.  I'd expect to pay no more than 500-800/yr for a mid range 172 and really good coverage.  The 12k cited in the Mooney example for a new pilot in a medium twin is an extreme example.  You won't see that.
A 172 should cost no more than 1000 for just the annual inspection. Parts and additional repairs can put you over 1k, but the inspection itself shouldn't be a bear.  Most 172 owners I know escape with sub-1k bills.  Shoot for 55-80/hr for labor.  Now, extra parts and other uncommon (but necessary) maintenance may add to the total cost.
Budget fuel at 50hrs/yr AS AN ABSOLUTE MINIMUM.  Plan for 75-100hrs/yr if you live in a humid environment (like the gulf coast or mid west).  Consider this a maintenance expense.  You MUST FLY the plane.  Letting it sit sounds cheaper...it's not.  Parts dry out, corrosion sets in, maintenance cost sky rocket.  A flying plane is a happy plane.
A Cessna 172 engine (Continental 300, Lycoming 320/360) will cost a pretty penny to overhaul.  18k cited in the Mooney example is a bargain!  Count on 25k minimum including removal and replacement of the engine.  Spread that cost out over 2000hrs for cost planning.  It's reasonable to expect a well maintained engine to run happily beyond the overhaul recommendation number (called TBO)...which is mostly just a BS number.  Flying beyond TBO is standard practice; I planned to run my IO-520 up to 2500hrs if it let me.  TBO for my engine was 1700hrs.  Every now and then a low time engine goes bad and needs an overhaul.  Ya' pays your money and ya' takes yer chances!
A fixed pitch prop is a low-maintenance item.  Maybe plan a few grand for an overhauled prop if yours becomes unservicable.
DO NOT fall for the glitzy avionics.  There is nothing wrong with a pair of King or Collins radios and an older GPS unit.  The fancy cockpit panels are nice, but are more for show than function in my opinion.
To answer your question...I'd budget 12-15k per year to own a decent Cessna 172.  High cost areas (like NYC, San Francisco, Chicago) may see that bleed into the 18k-20k range.
My planning:
-2400 per yr hangar (not covered parking)
-800 per yr insurance (1m smooth)
-1200/annual (plan for minor maint issues)
-2000 per yr fuel - 5 dollars/gal at 8gph
-800 per yr engine & prop (added some padding)
-1000 per yr for misc work
-2400 per yr (assuming a 200 per mo payment)
...and we're about 11,000/yr without breaking a sweat!  Toss in an unplanned maintenance event, like a starter or battery or alternator or magneto or cylinder or (insert component) and you're crossing 13k real quick.  Now, that does include 50hrs of flying, so you got that going for you...which is nice.
Please note I only pay 200/mo for a hangar (large city of about 1m people).  I'm absurdly fortunate.  The same hangar at a nearby airport is 450.  Might be over 700 in the bay area.  Needless to say, plan the budget with real numbers from shops, fuel providers, and hangar providers at the airport you intend to fly from.  Call a few insurance brokers and get real quotes, too.

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Do other countries create the equivalent of a TFR when US VIP's visit?
In the US, there's a TFR everywhere a designated VIP (US president or vice president) is going to be. When (most?) foreign VIPs visit the US, I don't think there are TFRs in place for them (unless the location coincides with our VIPs).
Are there TFRs (or international equivalents) in other countries when the US VIPs are there?

yesterday (25-03-2014) when the president arrived in Brussels Airport airspace was cleared. http://www.deredactie.be/cm/vrtnieuws/buitenland/2.32602?eid=1.1919218 (dutch link),

I don't know. I was at Andrews AFB once pre-flighting when the King of Saudi Arabia on his 747 flew in and we were grounded. There's definitely some sort of restriction when foreign VIPs come in.

I imagine this has a lot to do with how much inconvenience the hosting country is willing to endure. (I would further posit that in many cases if the hosting country says "Screw you, we're not mucking up our airspace for your self-important behinds!" some foreign dignitaries would probably refuse to travel there, citing security concerns…)

The question title asks about the US creating a TFR for foreign VIPs, but the question body asks about foreign countries creating a TFR for US VIPs. Which one are you asking? Or both?

@Pondlife Sorry, I got confused and mixed up the title, I think it matches the question now.

Yes, there is. I remember last time when US president was in Prague, LKPR was closed for quarter or half an hour around his arrival.

Absolutely!  The same airspace restrictions that are in the US are regulated by the FAA and ICAO.  The FAA (or the US) is a signatory to the ICAO standards, and as part of that governing body instituted the same rules worldwide. Those restrictions are also issued through the NOTAM system.
Incidentally, flying in the UK, we had restrictions on where we could fly depending on where the Queen was flying (if our missions coincided with her flights).  They even had a rather unflattering name for those flights in the pilot community.
Here is an article that talks a little bit about th e roles and responsibilities of the Queen's pilots.
You can also check UK NOTAMS (although i don't see any current ones fo rthe Queen at the moment).

I think the question was about US VIP's (e.g. the president and the Vice President) in international airspace like the UK. So, when Air Force One flies into the UK, will there be a UK NOTAM?

Doh! My bad!  I'll edit. And yes, NOTAMs are issued worldwide.

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What qualifications are required for an authorized flight instructor to teach IFR in AATD simulators, in the U.S.?
I'm from Brazil, and I am working on an IFR certification process via AATD Training Course. We are having problems with the flight instructors; they must have valid IFR and CFI licences.
The instructors are pilots with more than 10000 hrs, that have already flown heavy jets and executive jets.  Our national agency is not accepting our training program because they don't fly anymore and they don't have valid licences.
I would like to know how the certification process works in other countries, such as the US or in Europe.

To provide instrument instruction the instructor must posses a valid CFI certificate with an "instrument airplane" rating.  This is irrespective of the type of training device or airplane used with the caveat the instructor needs to be rated for the airplane (either flown or being simulated).  In other words, if you cannot instruct in an airplane, you cannot instruct in a training device either.
The applicability of using the AATD is more of a program certification than an instructor pilot certification issue as there are different levels of training and special certifications for certain levels of training devices and simulators.  See 14 CFR 61, 141, 142 for details about training programs and AATD use.

I'll dig up some references to link to when I have some time.


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Where can I find regulations in different countries for flight level rules based on direction of flight?
I'm from Brazil, and here we use the West/East rule, so we use an odd flight level when we fly between 0/360 - 179, and when we fly between 180 - 359 we fly in an even flight level.
But what should you do in other countries? Where I can find those rules?
I've heard that in Europe it's totally different, and that in some countries in Asia they use meters, instead of feet.
Where can I find this information?

You should be able to find this information in the AIP of the country in question, but were you looking for a place where this information was collated together?

Yes, this can be found in AIPs, but there is some place that i can find everything organized? Because, i imagine how difficult should be to airlines set the routes for the pilots, with the step-climbs at the flight plan.

Interesting fact: in most European countries the transition altitude is around 5000ft which is a surprise to many FAA pilots.

The transition altitude can even be as low as 3000 feet. There has been talk of standardisation for some time now. Unfortunately, it requires European countries to agree on something trivial.

Every country uses feet for altitude.

Negative, the Russians and Chinese use meters. An interesting MD11 feature, is that you can set on the MCP both measure units (feets or meters), so is easier to deal with the flight levels. Other airlines that fly in that regions, give conversion tables for the pilots, so there is a lowest possibility of errors.

http://www.air-child.com/media/docs/Flightlevel_Conversion_Sheet.pdf
Take a look, is used for flight simulation, but it gives a nice perpective about the flight levels.

Just to let you know, you should type the user's name to send them a reply; it gives them a notification - like this: @xpda

@YgorMontenegro - You're right, my mistake. Except Russia and most former Soviet states started using feet above transition altitude in 2011 when they started using RVSM.

Every country's Civil Aviation Authority issues an Aeronautical Information Publication (AIP) as part of their Aeronautical Information Services (AIS) which contains the information you are looking for. Most likely the information about Flight Level rules will be contained in the GEN (General) or ENR (En-Route) section. For Europe, most information from the AIP's can be found through the European AIS Database


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Why is the generator turned back off for starting the second engine on a King Air?
The engine start procedure for the Beechcraft King Air B200 says (emphasis mine):

Right Ignition and Engine Start Switch – ON [R FUEL PRESS] annunciator – OFF
Right Condition Lever – LOW IDLE (after N1 rpm stabilizes; 12% minimum)
ITT and N1 – MONITOR (1000 oC maximum)
Right Oil Pressure – CHECK
Right Condition Lever – HIGH IDLE
Right Ignition and Engine Start Switch – OFF (at 50% N1 or above)
Right Generator – RESET, then ON. CHARGE BATTERY until loadmeter reads approximately .50, then OFF
Left Ignition and Engine Start Switch – ON [L FUEL PRESS] annunciator – OFF
As Left N1 RPM accelerates through 12%:

Left Condition Lever – LOW IDLE
Right Generator – RESET, then ON

ITT and N1 – MONITOR (1000 oC maximum)
Left Oil Pressure – CHECK
Left Ignition and Engine Start Switch – OFF (at 50% N1 or above)
Left Generator – RESET, then ON
Right N1 – REDUCE TO LOW IDLE

Why is the right engine's generator turned back off before starting the second engine (step 7) and then only turned on again after injecting fuel (step 9)? Doesn't this put unnecessary load on the battery?

Prior to serial number BB1444 the Generator Control Unit (GCU) design and the associated current limiting system was not able to handle the load of a cross generator start. So we have the generator 'assist' procedure instead (to prevent the current limiters blowing during the second engine start).
From BB1444 onward they changed the design so that the current limiters (CLs) would not potentially blow and therefore later 200's went to a cross generator start procedure (allowing the pilot to keep the first generator on).
Tom Clements wrote about this in his book 'The King Air Manual' and you can find a discussion about it on KingAirMagazine.com:

[...] the GCU limits the output of the operating generator whenever the opposite start switch is activated. This current limiting began with the 300-series, but was added to the 200-series at BB-1444. [...]
Leaving the generator on increases the chance of experiencing a current limiter failure during the cross-start on the earlier airplanes, whereas doing the Off-On procedure does not harm the later aircraft in any way.

If you haven't read his book I highly recommend it.

Yes it loads up the battery, but the battery is sized to do this. The generator is not necessarily sized to do this, especially on the ground, so this procedure gets you to take the generator out of the system for each start.
On an Alternator, like on your car, there is a phenomenon known as 'forward-stripping', which is where there is such a demand for current out of the generator that it overloads the rectifiers (that convert the created AC electricity into DC for output). So here, during/after the engine start, there may be a large charge request by the battery which might forward-strip the gen's rectifiers.

@Bianfable You're not obligated to accept an answer if it isn't helpful to you. If you don't fully understand it, you can address the author (as you have) and leave it unaccepted. If somebody who could explain it better were to come by, seeing an accepted answer may dissuade them from answering.

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Where exactly are the heating elements in the PA-28-180's pitot mast?
I am flying a PA-28-180 but have been unable to find where inside the pitot mast the heating elements would be.
I would like to know this to verify my assumption it does not work in the airplane I am flying. The Pitot-heat has been on for about 40 minutes while flying through 5C weather, upon landing I checked if it was at least warm and it was not.
Can anyone verify or reference me some documentation of where and if you would be able to feel the heater?

I also fly a PA-28-180 but I don't have Pitot heat on it.  I am thinking that this is an after market add on for your particular plane which means there will be no documentation for it in the POH or parts manual.  There should be documentation in the maintenance logs about the installation of the Pitot heat element.  Did you check those?

I did not think that far ahead, the owner did not notify me it was an aftermarket upgrade (not sure if he knows). the POH does show pitot heat. I will definitely also check the maintenance logs.

You don't actually need to get into the mast to know if the pitot heat is working. I fly a PA-28 and during your preflight check if you engage the pitot heat and allow it to run for a minute or two the pitot tube should be warm to the touch. If it is not your pitot heat is not working (or is not reliable enough for flight).

Yeah I figured, thanks! I just really didn't know if it was 'feel-able', like in a pitot tube. I will notify the owner appropriately! thanks Dave!

It's super hot.  I got 2nd degree burns on my palm from gripping a heated tube on a 310.  It was a hot VFR summer day...I have no idea why the pilots were using pitot heat.  If the Piper elements are anything like Cessna's, only about 30 seconds of run time is needed to warm the mast.

To check a pitot heat, you DO NOT grip it with your palm and fingers - they'll burn onto the probe/mast. - Just stroke the probe/mast with the BACK of your hand/fingers, and you'll feel the heat without damaging your hand.

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How much fuel did the N Class Navy blimp carry?
The Navy N Class blimp was powered by 2 Wright R-1300 Cyclone 7 radial engines and was able to cross the Atlantic and back again without refueling.  How many gallons of fuel did it carry for a maximum range cruise?  The information is needed for a ton mile/gallon efficiency calculation.

Wikipedia says 200 hours endurance, and two R-1300s running at 50% power are going to use about 360 lb/hr  (800 hp x .45 lb/hp/hr for air cooled piston engine), or about 60 US gal/hr.  For 200 hrs endurance this would require... yikes... 12,000 gallons.  Maybe max endurance is dialed back even more, and the 200 hrs comes from a 40% power setting.  In any case, it's gonna have to have upwards of 10000 gal on board to run that many hours.  Hope someone's able to find an actual number from a specification somewhere.

@John K it's a start.  They may have "sailed" it with prevailing winds on one engine.  The Graf Zeppelin could lift around 190,000 lbs.  So 70,000 lbs minus 20000 lbs crew, provisions and equipment leaves 50,000 lbs, or around 7500 gallons fuel.

Well you would have to figure any range number would be based on still air and we're talking about a time aloft anyway in this case.  The GZ was much bigger no?

The 785 foot Akron carried 20700 gallons of fuel.  There is good published info on these.

@John K,  will work with Akron and Macon data.  Thanks for helping me dig these out. Would think the Cyclones on the N Class probably ran at a much lower RPM to get 60 knots than they would for a cruising airplane.


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What are N1 and N2?
When I'm reading something about jet engines, it's common to see references to N1 and N2.
I've never fully understood what N1 and N2 are on engine instruments and how they relate to thrust and the position of the throttle.

why is it called "N"?

@erich - It's been a year, but in case anybody else wanders by... "N" = "Nominal", see the first sentence of Airman01's answer.

N3 for RR and other 3 shaft engines.

Conceptually, N1 is the fan (or, fan speed) and is most related to your thrust (since the fan produces more thrust than the core on today's big motors). The N2 is the engine core, and the N2 gauge is used mainly during engine start: on initial starter engagement, the N2 starts turning first, and things like adding fuel+ignition and then later disengaging the starter are based on specified N2 speeds. As long as you have N1 rotation before lighting the engine off, N1 isn't all that important during the start. But once it is running, power is generally set with reference to the N1 (or fuel flow or EGT or EPR) rather than the N2.

This picture of 727 instruments shows, from top to bottom, the N1, EPR, EGT, N2, and Fuel Flow indicators.  You can tell (hopefully) how the N1 and N2 are both tachometers, showing percentages instead of values, because it's a lot easier to talk about 52% N2 than 6350 N2 RPM.  The picture below shows an example of the engine stack displayed on a modern EICAS panel, with the N1 and the EGT displayed prominently, and everything else just shown as digital values (with no EPR indication in this aircraft).  This is a typical in-flight presentation, showing you the two things you most care about with large displays, and everything else available if you really need it.

Nice answer, thank you. Why would the landing gear have an "off" position? Is that for emergency use, such as gravity deployment?

You should probably start a new question for this, however... with the landing gear control lever in the "OFF" position, hydraulic pressure is
blocked at the selector Valve. Both the down line and the up line are connected to return. OFF is the normal position in cruise. Boeing has 'OFF', Lockheed has 'Neutral'. Douglas has an 'Uplatch Check' position.

N1 and N2 are the rotational speeds of the engine sections expressed as a percentage of a nominal value.
It is similar to the revolutions per minute (RPM) of a piston engine, expressed in percentage instead of in the actual rate of rotation.
They are different because they are characteristic on two-spool engines (see the figure below). The first spool is the low pressure compressor (LP), that is N1 and the second spool is the high pressure compressor (HP), that is N2. The shafts of the engine are not connected and they operate separately.

Image credit: Emoscopes / Wikimedia

It's dependent on the aircraft and engine installations but these refer to the rotation speeds of the engine spools.  As listed above, usually the N1 refers to the spool which the fan and LPC/LPT are on and N2 is the HPC/HPT spool in the gas core.  The displayed values for N2 and N1 are often given as percentages of their maximum rotational speed.  Therefore the HPC spool will often display as running much faster than the LPC/Fan spool.
This isn't always the case; For instance the engine speed gauges in the Aero Vodochody L-39 display the HPC spool as '1' and the Fan/LPC speed as '2'.

I agree with Aero Vodochody L-39 ( to display the HPC spool as '1' and the Fan/LPC speed as '2') because , conceptually HP Core of the gas generator is the " primum " from which all the subsequent generates .This is one of the rare case in which I disagree with the protestant logic of the protextant world !


text:
Is there a preferred turn direction for helicopters?
If I have a requirement to fly a helicopter in a circle or racetrack over a point of interest that's stationary or moving relatively slowly, is there any reason to prefer left or right turns?
I'm not concerned about visibility and I'm not asking about turns in a landing pattern: I'm asking if - all other things being equal - there's any mechanical, aerodynamic or procedural reason that pilots would prefer to turn in one direction over the other.

Related, perhaps duplicate: Why would a helicopter climb in a spiral?

Who needs to have the best view?

Somewhat related, perhaps, @CGCampbell, but definitely not a dupe.

(Responding to your edit) If visibility isn't your objective then what is? That will probably determine the answer. Or are you asking that if all other things are equal, is there a preferred turn direction in helicopters for 'built-in' reasons of aerodynamics, torque, or whatever?

There some aerodynamic effects but insignificant compared to the need for lookout.  Quite involved but I'll try to add an answer over the weekend.

Most helis make CW turns because the pilots sit on the right and it is easier to judge position and drift when you can look out the window directly.  Turning CCW is no problem however but it takes a tad more concentration.
one drawback of CW turns from the right seat is that the ALT and ASI would be out of your field of vision, so you'll have some head turning to peek at them. In a CCW turn, its easy to glance at the instruments in the center console.

Aerodynamically, if performing ground reference turns in a no-wind condition, there is no difference. On the other hand, if in a moderate to high wind condition, Loss of Tail Rotor Effectiveness can be a real danger. Many accidents have been attributed to LTE. LTE can be encountered by way of several different mechanisms, one being turning (orbiting) in prevailing wind at lower speeds. Effectively, main rotor vortices are blown toward the tail rotor at an angle which blanks the inflow to the tr, degrading its effectiveness, potentially to zero. Pilots can mitigate the effect by turning in the direction of the rotor (as viewed from top looking down). There are countless articles and video presentations on the www. Search for Loss of Tail Rotor Effectiveness LTE.

I am adding an answer as the outer 2 are incomplete. [Simon obviously forgot to add his answer].
Vision is a valid reason, but only In confined areas [think valley or other aircraft]
I will attempt to explain the aerodynamic reason this only apply to very slow speeds or very sharp turns.
If the turn is to sharp and slow, the wrong way and the wind is in a dis-favorable direction, it is possible to "run out of" power peddle meaning there is not enough anti-torque force to stop the turn resulting in a spin and crash. Most flight manuals recommend turns into the direction of the "power peddle" to avoid this.
Demonstration of a sharp turn power side vs non power side.

note how the first turn is a wide turn away from the [power peddle] and the second is almost a stationary spin [into the power peddle].

I have seen turns in both directions. It probably does not matter anyways which way the turn is going, but if it is a single main-rotor helicopter, going with the counter-rotor spin is probably a little more effective at saving fuel although there is not much overall advantage either way if you are not looking for visibility.


text:
Do you have to explain what your emergency is?
Story Time:
Let's assume, for the sake of the question, that you were piloting a twin engine plane and the right engine, out of the blue, shuts down.
After realizing it you declare emergency, and explain very briefly to the ATC what is your situation at the moment, and after that you start to work out the checklists (I don't know the right order to proceed on a emergency scenario, but i don't think that it's important to this question).
After that you managed well the situation and safely landed the plane.
So here is my question:
At the moment of the emergency you weren't able to explain with details everything that happened to the ATC.

According to FAA, is it required to detail/explain (maybe file some kind of official document) your emergency to some kind of authority?
If you do have to file an explanation, what information do you have to supply?
Also what would happen if they "didn't believe" that situation was an emergency?

You don't even have to declare an emergency to get one. I have had to incidences were they ask if I was declaring an emergency. In one instance I said no, in the other case I said I was making a precautionary landing. In both cases ATC went ahead and declared an emergency anyway.

The order is "Aviate, Navigate, Communicate".
As a pilot, your first responsibility is to keep the plane flying. After that it is to avoid hitting something or to avoid getting lost. After that comes communication. There are many instances of in-flight emergencies where the pilots never talked to anybody about it, because they were busy flying the plane...

According to FAA, is it required to detail/explain (maybe file some kind of official document) your emergency to some kind of authority?

The FAA has many procedures on reporting accidents, incidents, and emergencies. Whether or not you report it depends on what the emergency was. If its a very sick passenger, there may be no reporting required. If your control cables failed, you are required to report it. I'll update this answer with the regulations as soon as I find them all. Quick answer is any time you declare an emergency or "PAN-PAN", call the FSDO and let them know. Also depending on the nature of the accident/incident, you may be required to immediately notify the NTSB. See CFR 49 830.5

If you do have to file an explanation, what information do you have to supply?

This depends again on the nature of the emergency, and the FSDO/NTSB will ask you for the proper information when you file the report. At the very least they are probably going to want a narrative of what happened, and depending on what took place, you may want to contact your lawyer first. (Note: You may have to contact the NTSB immediately but it doesn't say you need to provide details, just that there was an incident, so the NTSB would be your first call "I had an incident", short and sweet, and the lawyer second.)

Also what would happen if they "didn't believe" that situation was an emergency?

Its not ATC's responsibility to determine if you are lying or not. When you as a pilot say "emergency" you have their full attention. After you get on the ground and the emergency turns out to be you forgot to call your girlfriend/wife and wanted to get ahead of other traffic, then the FAA will get involved. Until you are on the ground, an emergency is handled as priority, regardless of the reason.

Here you can read a good story about a stupid pilot who abused the "emergency" system by landing on a public beach in New York because of an airsick passenger and an engine that was "running a teensy bit rough". He didn't officially declare an emergency, and I can't find any record of official enforcement action against him for doing that. Note the NTSB did not investigate this "incident".

As I read your response to the quote about 'believing' the pilot, I thought to myself "Pilots know that ATC isn't some locker-room joke-fest; no one would frivolously declare an emergency." Then I read your last paragraph lost a small amount of faith in humanity.

Its not ATC's responsibility to determine if you are lying or not. When you as a pilot say "emergency" you have their full attention. aaand then there's this

So did the silly pilot actually PAN-PAN or did he just take it upon himself to land on the beach?

@OrangeDog As far as the reports I've read are, he said the words "precautionary landing" and ATC considered it an emergency and dispatched police, so while the pilot didn't declare it, ATC basically did it for him.

@TomMcW Thats a good example of how to get disciplinary action being a controller. While the pilot didn't properly identify himself he certainly knew that there was a plane on the runway evacuating and still questioned the issue.

"Aviate, Navigate, Communicate", that's the important bit. And the fact that in all cases the pilot is the decision maker. ATC is just there to facilitate. If you say "I need to land now" ATC should "drop everything" and help you land safely. Once your on the ground it could get interesting.

@RonBeyer, the NY-times article is paywalled

@ROIMaison it was not pay walled 6 years ago, if I can find another source I'll update.

@RonBeyer Thanks, seems like the walls are advancing :)

Per 14 CFR 91.3, you only have to provide more details if the FAA asks for them:

§91.3   Responsibility and authority of the pilot in command.
[...]
(b) In an in-flight emergency requiring immediate action, the pilot in
  command may deviate from any rule of this part to the extent required
  to meet that emergency.
(c) Each pilot in command who deviates from a rule under paragraph (b)
  of this section shall, upon the request of the Administrator, send a
  written report of that deviation to the Administrator.

But per 49 CFR 830.5 you have to notify the NTSB immediately (instructions here) if various specific incidents occur:

§830.5   Immediate notification.
The operator of any civil aircraft,
  or any public aircraft not operated by the Armed Forces or an
  intelligence agency of the United States, or any foreign aircraft
  shall immediately, and by the most expeditious means available, notify
  the nearest National Transportation Safety Board (NTSB) office when:
(a) An aircraft accident or any of the following listed serious
  incidents occur: [...]

Engine failure in a large multi-engine aircraft is only reportable if two or more engines fail.
In the general aviation world, if there's no damage to people or property then incidents or accidents rarely lead to any action from the FAA and they mostly aren't reported to the NTSB (because they don't have to be). Filing an ASRS report is often a good idea, especially if the FAA does decide to investigate.
I have no idea about the airline world, but it's safe to say that any incident is taken a lot more seriously and handled a lot more formally, even minor ones. Although all in-flight emergencies are handled in the same way by ATC, whether they're airline flights or not.

"Upon the request of the Administrator", means that ATC has your tail number and passes it to the FSDO, the FSDO can contact you if they want more information. So you are not required to contact them other than in response to a request.
Accidents and serious incidents are totally independent of your use of the emergency clause to deviate from the default operating rules.
Minor incidences in airlines are generally handled internally, they have roughly the same reporting requirements as GA, however their internal handling of an incidence is dictated by FAA approved operational specifications.

My understanding in the US is that controllers will frequently consider an emergency that is handled well and has a successful outcome as a non-event.
Among the reasons:

Less paperwork for the Pilot
Less review of the controllers work!
(Once the emergency is reviewed, the smallest mistake in phraseology can come up)
It encourages people to fly safe by letting them declare an emergency without any consequences.

I've personally seen a pilot declare an emergency because:"One cylinder is running a little hotter than the others".   He landed just fine, and didn't have to file anything.
Its better to declare an emergency and get extra attention in case the situation gets worse than it is to avoid declaring just because you're afraid of paperwork, and end up dead.

"declaring an emergency" is mostly a CYA situation and as a courtesy to help ATC know to expect non-normal actions,(and so they can provide extra assistance. There isn't an explicit regulation about declaring it, 91.3 does not say anything about declaring one, only that the the pilot may deviate from normal op. rules at their discretion to the extent required to mitigate the perceived hazard. Now, refusing to give ATC info when you have a functioning radio and moderate workload may be considered outside the "extent required", but this is only implied by an extension of other operating rules.

Here in the US under the FAA if you say something to the extent of

N12543 Declaring an Emergency

You have declared an emergency, no further explanation is necessary while you are in the air, they will always trust you and you will have priority over all other aircraft. Since you are now an aircraft under duress you can ask things of the controller that you may not otherwise be able to ask like "please clear RW24 for a straight in approach" or "where is the nearest airfield and can you clear the runway" etc.
Often times in declaring an emergency its in your best interest to provide a brief description to the controller for your own safety. For example if you say

N12543 Fire in the Cockpit where is the nearest airfield?

They will dispatch a fire crew (if available) or any similar situation that may require emergency services.
Now you are on the ground safely, there is some stuff you need to do. You should contact your local FSDO to report the accident then you must file a report with the FAA/NTSB you can find the instructions on how to do this here. The full form can be found here and is fairly verbose (it outlines what you need to report/the kind of explanation you need to give). Once this has been done the NTSB will investigate the incident which depending on the nature of the incident can take time.
I was always taught in flight school to err on the side of caution when it came to declaring. That falls in the "better safe than sorry" mind set.

air → err? (I know this is aviation, but still ;-) .)

And by "the side of caution" you mean, "when unsure, do declare", I presume?

Yes exactly, generally (unless is something really outlandish) you wont get blamed for declaring even if its something small.

From my knowledge, all you have to do to declare an emergency is to announce it over the radio.
This gets you priority for the traffic controller responsible for your area. They may further ask you questions, to coordinate any emergency services that you may require, or to further help them in their duties guiding you.
For example, if you have a co-pilot incapacitated they may put further spacing between you and other aircraft in the sequence in order to give you more time to go through the landing checklists and procedures.
Or if you have a medical emergency they may ask in order to inform the airport to ready the emergency services; or if a mechanical issue they may coordinate with the airport for a full stop on the runway, etc.
However all of that is strictly optional. As in an emergency situation your first priority is to fly the aircraft; therefore ATC will understand if you are not very chatty on the radio, etc.

I'm going to keep this short and sweet because other answers have caught most of the information, but everyone appears to have missed this minor detail:

§91.183    IFR communications.
Unless otherwise authorized by ATC, the
pilot in command of each aircraft operated under IFR in controlled
airspace must ensure that a continuous watch is maintained on the
appropriate frequency and must report the following as soon as
possible—
(a) The time and altitude of passing each designated reporting point,
or the reporting points specified by ATC, except that while the
aircraft is under radar control, only the passing of those reporting
points specifically requested by ATC need be reported;
(b) Any unforecast weather conditions encountered; and
(c) Any other information relating to the safety of flight.

This only applies to IFR flight, and one could argue that the emergency itself could cancel out this very rule by power of 91.3(b).
It's almost like a grandfather paradox.

I was taught that rule meant I had to notify ATC about anything that could affect the safety of other aircraft.


text:
Do modern airliners have oxygen sensors in the cabin?
I know that airliners have pressure sensors to check the cabin pressure, and thus check if we can breathe (and get enough O2) inside it so we don't get hypoxia. But that got me thinking:

Is there any oxygen sensor to check if there is enough O2 inside the cabin?
Is there any situation (emergency or not) in which some other gas could pressurize the cabin and lower the O2 percentage inside it?

you'll need other sensors to make sure the air is breathable. CO2 for instance becomes a major problem long before it's replaced all the oxygen in air.

You cannot inject air without the normal ratio of oxygen in the cabin (as air comes from the outside), then, if we except fire and decompression, the only possibility to have non breathable air is by not eliminating other pollutants, then @Hobbes is right, it's not oxygen that must be measured.

There’s a recent incident on AvHerald potentially caused by air quality issues

Why did I just think of canaries in a coal mine?

The air conditioning system on board airliners is an open system: it scoops up air from the atmosphere, compresses it, expels it through the cabin vents, and then outflows it through discharge valves. In modern airliners there is a certain amount of re-circulation: the expelled air was of such good quality that it seemed a waste to just dump it in the atmosphere.
Image source
In the A320 family of airliners:

Outside air is compressed in the engine compressors, part of it is tapped of as bleed air: already compressed and warmed, it is used for refreshing the cabin air (marked green).
Temperature and pressure regulators then provide automatic mixture control for the amount of outflow: the outflow valve is regulated, decreasing the opening results in increase of cabin pressure and decrease of flow.
The recirculation fans regulate how much air is re-used, after being filtered through HEPA filters which remove pathogens and other nasty stuff.

The whole cabin air volume is refreshed every 2 - 3 minutes, as also referenced in this answer: with such a frequent refresh rate, it would be pretty safe to assume that there will always be roughly the percentage of oxygen in the cabin as there is in the outside atmosphere (see below for the actual percentage!) . People have climbed to the summit of Mount Everest without the help of oxygen masks, and that summit is about at cruising altitude of an airliner. AFAIK, the aircon system does not have oxygen sensors, but it does have temperature and pressure sensors which are of more immediate concern.
Low concentration toxic gases are much more dangerous than a drop in oxygen concentration. In fact, the human body makes use of this: when holding your breath, the urge of breathing fresh air again after a while comes from the increase in CO$_2$ in the body, not from any decrease in oxygen. So CO$_2$ detectors on board of aircraft would make sense, as would detectors for the very lethal carbon monoxide. These are available, even in the form of a key hanger for GA pilots (not including any links due to them being adverts).
There have been reports of dizziness and fainting on board of airliners, and investigations into these. A sample of various flights did not reveal a particularly high carbon dioxide content:

Discussion
With additional measurement reports pending, I found in-flight cabin air quality measurements of oxygen to be relatively stable, ranging from 11.2% to 12.5%.
Typical outdoor CO2 levels are between 350-400 ppm (0.035% - 0.04%) or up to 500 ppm by some sources.
Carbon dioxide levels measured in-flight in the aircraft cabin ranged between 0.04% or 400 ppm and 0.1% or 1000 ppm to date in our studies and were measured at close to 0.5% or 5,000 ppm in earlier studies.
As indicated at CO2 HEALTH EFFECTS, occupants are unlikely to be affected or to notice CO2 levels under 2% or 20,000 ppm - a far higher number than in-flight aircraft cabin carbon dioxide levels.

Notice that the oxygen contents on board is a bit over half of that of the outside air - the cause of dizzy passengers? Reports of dizziness do keep appearing, and nothing beats direct detection of noxious gases by a sensor. Problem is, there are so many of them. Oxygen, carbon monoxide and carbon dioxide sensors would be good candidates for inclusion into the aircon systems.
On your second part of the question

Is there any situation (emergency or not) in which some other gas could pressurize the cabin and lower the O2 percentage inside it?

Yes, in air conditioning systems that use bleed air there definitely is, an engine malfunction can for instance inject gases from burning oil into the cabin air, a problem that was reported a few times for the BAE 146. From this report:

The British Aerospace BAe 146 was the aircraft type most commonly involved in fumes/smoke events when taking into account flying activity. The Airbus A380, Boeing 767, Embraer EMB-120 and E-190 were among other aircraft types that also had a higher than average rate of fumes/smoke occurrences over the period.
The most common source of fumes/smoke was aircraft systems issues, primarily relating to failure or malfunction of electrical and auxiliary power unit (APU) systems. Equipment and furnishings also featured highly as a source of fumes and smoke. Within this category, air conditioning and galley equipment were the most common sources of fumes/smoke. External sources of fumes/smoke and cargo/baggage related events were relatively rare.

"People have climbed to the summit of Mount Everest without the help of oxygen masks, and that summit is about at cruising altitude of an airliner". I would suggest you delete that part. Some very prepared people have survived climbing Everest without oxygen; many have died, and a percentage of people die of altitude sickness even at heights 75% of Everest. Also, airliners fly 25% higher than Everest.  And, in any case, all airliners pressurize to about 2000 meters, so the point is moot.

Also the people who attempt Everest, with or without oxygen, spend weeks at base camp acclimatizing, and the slow yak-trail trek to base camp is its own acclimatization.    Base camp is at 17,600 feet.  It may not be humanly possible to skill up to being able to breathe at MSL and then 40,000 feet an hour later.

@MartinArgerami The reference is included as an Order Of Magnitude check for assessing if we need to do anything additional before compressing it for cabin air. The proof is in the summit. Nowhere did I suggest that it would be a good idea to expose Aunty Mable to unaltered cruise altitude air.

While it would not significantly affect the O₂ percentage one thing that must be tabulated on a cargo aircraft is the amount of dry ice loaded. Dry ice is very common in packaging to keep perishable shipments cool during transport. Since dry ice sublimates into gaseous CO₂ as it warms there is a limit to how much can be loaded into an aircraft. If there is too much there's a possibility that the pilots could be overcome by gas trapped inside the plane. It's a rather insidious threat because you can't smell it so you would only realize something was wrong when you started feeling physiological effects.
An O₂ sensor would not help since pilots would be overcome long before it made any detectable difference in O₂ levels. AFAIK there are no sensors for CO₂ either. The amount of dry ice loaded is carefully tabulated and pilots who fly cargo containing dry ice are aware of the possible hazard and how to recognize symptoms of CO₂ poisoning.

CO₂'s effect on humans is not just that it displaces oxygen: our lungs can only move CO₂ from our blood to the air if the partial pressure of CO₂ in the air is quite low.  Fun fact: CO₂ build-up in the bloodstream triggers the breathing reflex (not actual lack of oxygen).  High CO₂ is a problem even if there's also plenty of O₂ in the air.  See also https://en.wikipedia.org/wiki/Hypercapnia#Tolerance for the physiological effects of increasing CO₂ concentrations, and how long until you're incapacitated.  (Note this is CO₂, not the far more deadly carbon monoxide, in case anyone missed that.)

The system would be kind of superfluous, since the ambient environment around the airplane, and the cabin itself, is filled with air. The air in the atmosphere is a mixture of approximately 78% nitrogen and 19% oxygen by volume, so the known quantity of oxygen, and the partial pressure of oxygen, would be known, or relatively easy to calculate, if you knew what the cabin pressure itself was. As such there is no reason to put in an oxygen sensor within the cabin of an airliner.

While technically correct, there are (rare) situations in which air could become contaminated. See: https://aviation.stackexchange.com/questions/14664/do-large-aircraft-have-carbon-monoxide-detectors#comment34039_14667

There are a number of ways the air could become contaminated, but those largely don't involve any significant difference in the concentration of oxygen in the air, so an oxygen sensor wouldn't measure them.


text:
Why are landing gear retraction mechanisms mostly based on the four bar system?
I would like to know about the principles of landing gear retraction system and I struggle to find clear information about it.
It seems to be about a four bar systems in most cases. This seems more complicated than necessary at first glance, why is a design with a simple hinge with an actuator that pulls the strut in not sufficient?
Most retraction retraction mechanisms involve a lot of pieces with complicated motions. I would like to understand the kinematics that justify the use of more than one or two bars. Intuitively, one would only expect an actuator acting directly on the landing gear, holding it into position, but the reality always involves complex rotational kinematics.
This question would also apply to the kinematics of flap tracks, which seem to be fairly complex as well.

That's a pretty broad question.  While there is some commonality, landing gear systems can be vastly different.  Some retract forward, some sideways, and some backward.  Some rely on hydraulics, some on pneumatics, and some require manual force to actuate.  And most have some sort of overcenter lock to prevent them from folding back up under weight.  What specific "kinematic principle" are you interested in understanding?

This is a valid question that should not have been declared Too Broad. The undercarriage retraction mechanism is part of aircraft design.

You should provide examples so that we can answer for those specific examples of undercarriage retraction mechanism and extend to similar mechanism.

The main landing gear retraction system poses quite a few problems to solve:

The retracting actuator must have a reasonable duty cycle during the retraction. Due to trigonomical effects, retraction force can become very large - 1/cos $\phi$ for $\phi$ approaching 90° for instance.
The actuator stroke must remain within reasonable limits.
The retraction actuator and linkage must fold itself out of the way. Or be mounted at the other side of the wheel well, causing greater loss of useable wing space for fuel etc.

From Torenbeek:

Most retraction mechanisms are derived from the four-bar linkage and the designer must have very good reasons to deviate from this. A suitable pivot point must be chosen for the leg which, at the same time, gives the required wheel positions and allows adequate length of the leg. A retraction mechanism, generally consisting of a folding stay member and a retraction jack, is then required.

A picture from this video, which shows how the actuator retains a more-or-less constant favourable position and moment arm during the retraction process. This video shows the retraction of the A380 landing gear.Video
Of course, the exact configuration of a retraction mechanism depends on many structural details of the aircraft, and there are many different lay-outs. But the underlying principle remains the four-bar system in the vast majority of cases.


text:
How much oil did Charles Lindbergh have to bring along to fly the Atlantic?
A study of the Wright Whirlwind 200 hp J5 made me wonder if it could be matched to a Cessna 172 airframe.
Radials have the ability to run on much lower grade 50 octane gasoline and have great air-cooled reliability. The J5 can generate 200 hp at less than 2000 rpm.
But it was found these these engines burned far more oil than the Continental or Lycoming ones, with oil/fuel consumption ratios approaching that of 2 cycle mixtures.
For all the desire to save weight, the Spirit of St. Louis would have had to carry a considerable amount of oil to make the trip.
How much oil did Lindbergh take along, and where was it stored in the plane?

I read his book about the trip years ago.  The oil tank doubled as the firewall and was quite large and IIRC it had enough volume to supply the engine for the 39 hour trip. When Lindberg arrived over Paris. he had more than enough fuel left to make it to Rome, and mulled carrying on for a bit, but decided that crossing the Alps at night was pushing his luck a bit too far and started to look for Le Bourget.

Source: http://www.charleslindbergh.com/plane/
This site has a diagram of the plane:

And links to a PDF that states:

The 25-gallon oil tank was arranged between pilot and engine to as to act as a fire wall.

An oil tank as a firewall? Gutsy!

@ceejayoz - better than a gas tank. You can throw a lit cigarette into a pool of diesel and nothing will happen (well, the cigarette goes out). Oil is even harder to get to burn.

Plus a tank of oil, at least while it's full, will have a much larger thermal mass than any wooden board that would have been used in that era. I'm picturing a nice white blanket of woven asbestos fiber on the engine side, but most likely Lindbergh would have passed on that on weight grounds.

@JonCuster you can actually do the same with a cigarette in regular gasoline, though it's arguably more about how bad at cigarettes are at igniting vapours than anything else https://pubmed.ncbi.nlm.nih.gov/21605828/

I recall reading a book on the subject in the early ‘70s that included a drawing showing a substantial oil tank in the middle section of the wing, above the fuselage.  If both the book and my recollections are correct then there may have been even more oil on board.

@rclocher3 My significant other is a safety engineer for a petrochemical company. She demonstrated. There's plenty of demonstration available : https://www.youtube.com/watch?v=7soVqyGq4i4


text:
Why do the ailerons on an A320 droop when the engines are shut off and the 787 ailerons dont droop when the engines are shut off?
I have been wondering why the ailerons on the A320 (and other Airbus aircraft) droop when there is no power from the hydraulic pump. From my understanding of hydraulic actuators, the position where it was before the pump was shut off, is where it is gonna stay (if that makes sense) because you are closing the fluid intake and outake valves. I think I understand how Boeing actuators work, and why they stay in place with no hydraulic power coming from the pump. Just why do these Airbus ailerons droop with no hydraulic power?

ASE is visited by many new English speakers (since English is the language of aviation) -- perhaps consider replacing "gonna" w/ -- ??

The A320 aileron actuators have servo valves of course, which when closed indeed allow no flow through them from pressure to return. They additionally have mode valves, which do provide a direct (damped and narrow) connection, and it is this connection that allows for a slow and smooth aileron droop when unpowered.

Pic above is from this site, which contains a thorough explanation and simulation of the actuator. The right hand side is the valve in stand-by mode,

In active mode, the mode valve establishes connection between the servo valve control ports and the cylinder ports enabling nominal functioning of the system. In stand-by mode, the mode valve isolates the control ports of the servo valve and connects both cylinder chambers through a damping orifice and anti-cavitation valves – refilling the chambers in case the pressure goes below return pressure.

@JohnK added a valuable comment: the main reason to do this is, when the hydraulic system is unpressurised there is passive flutter damping...

...meaning fewer or no dedicated passive flutter dampers are required for that surface than if you have actuators that hydraulically lock with no pressure present and no spool valve displacement from a control input.

You could add that the main reason to do this is you can take credit for the actuator's idling circuit in providing passive flutter damping if its hydraulic system is unpressurized, meaning fewer or no dedicated passive flutter dampers are required for that surface than if you have actuators that hydraulically lock with no pressure present and no spool valve displacement from a control input.


text:
Can the rudder be considered part of the vertical stabilizer, and the elevator part of the horizontal stabilizer?
Or is it strictly correct to say these are separate elements, and you can't call the whole a "stabilizer"?

The control surfaces and the stabilizers are not the same. The free dictionary:

Horizontal stabilizer — The horizontal airfoil of an aircraft's tail assembly that is fixed and to which the elevator is hinged

To refer to the assembly, you may use horizontal / vertical empennage, empennage alone meaning both horizontal and vertical surfaces, synonymous of tail.

Stabilizers are stationary parts. Their role is to increase aircraft stability, that is the tendency to return to a stable attitude after a disturbance.
Movable parts, rudder and elevators, are control surfaces, they change the empennage camber. They are used to temporarily set a new attitude. When returned in the neutral position, stabilizers restore the stable attitude.

Empennage components, source
The third component, the tab, is used to trim the movable surface.

There are solutions other than the assembly stabilizer + control surface:

A stabilator (stabilizer + elevator) is a single movable horizontal plane turning around a median point. Therefore there is little resistance when adjusting it as a control surface. Some artificial feedback is added so that the pilot can feel a resistance proportional to the rate of deflection. See this question.

A trimmable horizontal stabiliser (THS) is a classic two-part horizontal assembly but where both parts are movable independantly. It allows to manage stability within an extended range of airspeeds without drastic changes in the overall camber.
The control surface is under pilot control, the movable stabilizer can be under control of computers managing efficiency, stability and flight envelope or under pilot control via the trim wheel (thanks to @Bianfable for adding this information).
When managed by computers it may return to a mechanical linkage when something is wrong with the computers (see Airbus flight control laws).

Quite a number of GA planes, for instance the Piper Cherokee, have a stabilator.  I don't know of any planes that have a rudder/vertical stabilizer designed on the same principle, but I don't know why it wouldn't be possible.

What is that "median point"? The choice of the rotation axis for a stabilator can be driven by many considerations, and not always does it have "little resistance". Remember it is most common in supersonic aircraft, not in GA.

@Zeus: This is detailed in Axis of rotation of tail surfaces in particular your answer deals with Mach tuck and swept airfoils.

If we are talking about the surface area, then yes: the moveable control surface area is defined as being part of the stabiliser area. For instance from the above definition, derived from NACA TN 775.
But indeed, the control surfaces do move, and deflecting them would alter the projected area. Similar to the lift equation, the control surface deflection is considered to generate a change in curvature of the surface, and therefore a change in $C_L$ and $C_M$ of the surface, and not a change in stabiliser area.
$$L_{CS} = C_{L_{CS}} \cdot \frac{1}{2} \rho V^2 \cdot S_{CS}$$
Index $_{CS}$ for Control Surface, with $L_{CS}$ and $C_{L_{CS}}$ changing as the surface deflects, $S_{CS}$ remaining constant as per the image above.
If we're not talking about area, it is indeed obvious that the control surfaces are not part of the fixed stabilisers - because control surfaces can move.
----Update----
NACA TN 775 dates from 1940, when a majority of aeroplanes were manually controlled. If the control surface can indeed "flap in the wind" as stated by @JohnK in his comment, how can it be that the moveable control surface area was included in the stabiliser area?
The answer is that is is more complicated, as commented by @slebetman. Stability considerations for reversible flight controls do consider stick-free static and dynamic stability. During changes in airflow angle and pitch/roll/yaw rate, the airforces exert a hinge moment on the control surface. The demand for stability is now that the change in hinge moment may not result in too much of a control surface deflection.
The following factors determine this:

Friction.
There is friction in the cable loop from stick to control surface, which prevents the surface from deflecting until the friction level is exceeded. In large aeroplanes the control loop friction can be considerable.

Mechanical springs. As mentioned in @JohnK's answer, a mechanical spring can be added.

Shift of the surface axis. Moving backwards the axis around which the surface hinges, results in part of the airstream wanting to re-align the control surface.

+1, for the view about total acting area.

There are additional nuances. If the control surface is free to trail because it has no trimming device to fix its deflection angle like a tab or bungee, like the elevator on a C-180/185 for example, it doesn't really contribute to stability in the stick free case; it just trails like a piece of laundry flapping in the breeze. Same with the rudder.  Cessna adds bungee springs to the rudders of 180/185s to add yaw stability on floats without having to add ventral fins, by partially "fixing" the rudder (within the bungee spring strength) making it contribute to fixed fin's weathervaning ability.

@JohnK Not quite true. The free-to-trail part still contributes surface area when it comes to calculating stability (static margin) of the CG


text:
Does a biplane need wing twist?
Does a biplane need wing twist?
I understand wing twist is used to:

creat a soft/partial stall ( hopefully create a partial stall before a full stall)
balance the wing so a smaller horizontal tail is needed with less drag.

So if the top wing on a biplane is canted upward, in a mild stall, it will stall first, with the bottom wing still flying, creating a partial stall.
If the bottom is at a lower AOA, it's like having wing twist on 50% of the plane. Wouldn't this act like wing twist on a monoplane and reduce the total moment, therefore needing less of a horizontal tail, like the self balancing wings on modern gliders. Is this correct, at least conceptually?

The reasons for twist mainly apply to swept wings, which would naturally stall tip-first (due to the outward flow velocity component) and that would create a very nasty pitch-up moment (as the root still producing lift is forward), so this needs to be compensated somehow, usually with twist. But straight wings don't produce this effect and many straight-wing monoplanes don't have any wing twist.

Wing twist is not required for any wing, although it is often used (as you said) to make sure that not the entire wing stalls at the same time, but also to tune the spanwise lift distribution.
Both of these are much lesser issues for biplanes:
Stall on a biplane
Any wing (when creating lift) creates a flow field around it, where the flow below it is slowed down and the flow above sped up, the flow in front is pulled up, and behind it pushed down. In a biplane, the upper wing is usually a little ahead of the lower one. This means that the upper wing sits in the sped-up region from the lower wing, and gets a higher angle of incidence -- that makes it quicker to stall once angle of attack increases too much. The lower wing is the opposite, though. It gets a lower velocity flow, and the lower side of the upper wing is helping to turn the flow, so it doesn't separate from the lower wing as easily. This means that biplanes have a naturally very gradual stall behaviour, which makes them much easier to control in near-stall conditions than most monoplanes. Triplanes are even more stable.
Lift distribution
The reason modern passenger aircraft use wing twist to control lift distribution is that it's easier than using wing planform. In the 1930s and 1940s, elliptic planar wings were all the rage because they're the aerodynamic optimum for planar wings, but they are not the overall optimum (structural weight, manufacturing costs...), and that's why modern passenger aircraft have wing planforms with straight edges, and lift distributions half-way between an ellipse and a triangle (the higher inboard lift can be borne easier because the inboard wing can be made thicker, and it causes less bending moment at the wing root). For bi-planes, they usually have struts between the two wings, so they're incredibly stiff and can bear much higher loads with equal structural weight and despite using quite thin profiles -- This, the small moment of inertia for rolling and the benevolent stall behaviour is why biplanes are still a thing in aerobatics. That's also why tailoring wing twist isn't as critical for biplanes as it is for e.g. large passenger aircraft: There's not as much to gain from it.
This would probably be different if somebody wanted to build a large long-range highly efficient biplane. In that case, there would probably be some gains to be made from introducing wing twist. However, unless that plane had extremely highly stretched wings, it would likely still have worse efficiency than a conventional one because the struts between the wings cause quite a bit of drag, and you'd still have to make relatively thick wing profiles to fit the fuel in, negating the other advantage of biplanes. And that's why such biplanes don't exist.

The Zeppelin Staaken Riesenflugzeuge had aspect ratios in excess of 10 and would come close to a long-range biplane. But even they did not have twist.

@Peter: nice planes... Those concepts certainly made sense at the time. I would think, though, that the potential efficiency gains from introducing wing twist to them would have been tiny compared to all the other improvements (like using monoplanes...) found between then and the 1950s -- at which point wing twist, rather than planform, started to become the method of choice to control lift distribution.

Yes, those giants produced horrible zero-lift drag – but were extremely light, which helps in low speed flight. The limited engines of that time did not allow for more.

Evidently, they don't. The picture shows a fully aerobatic Pitts Special from 1966 with no visible wing twist.

Small, light, aerobatic plane, no.  Huge aircraft that could not pull out of a dive in time following a full blown stall, might not be a bad idea.

Go back to the early 1900s and have a look at the "tailless" Dunne biplane.  This designer very ingeniously realized wing twist (washout) is possible, making stalling characteristics much more benign, and, taking advantage of wing sweep to use lower AOA aft wing tips to pitch the nose down.  This is the fore-runner of todays slats, which can be retracted in cruising flight to save drag.
Other early biplane designs did put the upper, forward wing at a higher AOA, as a stall safety measure, but now with two equally sized wings, one cannot be set to its most efficient AOA.
So, especially if you strive towards a higher aspect, more efficient wing, where do you put your pitch control?  Traditionally, aft, taking advantage of the longer torque arm of the fuselage.
Birds deal with this issue by fanning their tails open at high AOA for pitch control, then folding them at cruise to save drag.
But where else could it go?  Yes, forward!  Leave the empennage on and put another control surface forward.  Instead of having a bi-plane, a much smaller forward wing would serve for stall warning, passively dropping the nose (it would stall first).
So if you want tailless, you need something else to control pitch.  Lower aspect, washout,  placing trims and control surfaces closer to CG come at a price of lower stability and/or more drag.
Computers, which can trim several times a second, have made the reduction in size of passive pitch control "fins" possible, but their function remains critical for safety.  Washout and/or slats do add drag, but make the aircraft much safer in low speed/high AOA flight.
So, how about bigger, retractable slats (for swept wing aircraft), or a "safety canard" for all.

Or perhaps a more robust AOA measuring device.

havinig a forward control surface go into a stall could be exciting,

@Jasen Good point, that's why it would only be a wing. No control surfaces. Just something to stall before the main wing.

still a poential roller-coaster though.

That's why we keep the empennage.  A relatively small canard simply would drop the nose and unstall itself.  Moving the whole stabiliator rapidly is the roller coaster.  I'd rather close the barn door before the horses leave.

I love canards, especially as a pusher like the velocity. Instead of having this huge empty empennage on GA planes, the whole plane is used. The south Koreans redeveloped the velocity, a 4 place plane, with a useful  MTOW of 4,000lbs!! Someone did say somewhere, the problem with canards is that they produce wing tip vortices, so the main wing has to be bigger than a monoplane.

I understand many GA planes built of aluminum don't have twist for ease of manufacturing. They make all the ribs the same size. I understand the eppler self balancing airfoils don't have twist, so twist on a small plane made of fibreglass is outdated technology. Is this correct?

@Fred The "twist" is an insurance policy against tip stalling, which is especially problematic in tapered swept wing designs.  But even a Hershey bar can tip stall if not flown in a coordinated manner.  With canard designs, make sure it passes the "drop" test (nose must pitch down).  Read about the Army Air Corps Ascender.

Talked to the owners of Velocity and was told they want their canards to take 33% of the total lift to avoid deep stall, so they want high aspect ratio ajd


text:
Where can I find a free list of ICAO and IATA airport identifiers?
Is there an open source or free comprehensive (or close to it) list of ICAO to IATA airport identifiers? The official list from ICAO costs hundreds of dollars. I suppose such a list could be scraped from Wikipedia.

It depends on whether it should be reliable enough to be used operationally. If yes, I don’t know one and would to the best of my knowledge advise spending some sort of money. If no, maybe openflights.org could help?

If you don't mind a little leg-work, you can put together a very good list, although it may have some variance from the official product.
This site is oriented toward weather reporting and you'll have to edit out some "fake" identifiers that are simply weather stations without an IATA identifier, but that's a pretty straightforward exercise using a spreadsheet.
The site OurAirports.com has a .csv file you can download (top line menu "Data" then select the CSV download) which has a vast number of airports, with both ICAO and IATA identifiers listed.
If you combine the two data sources, you're probably 90%+ of the way to having the exhaustive list. If that level of completeness is enough for you, then you're probably good-to-go, especially if you include what was suggested in another answer and what you can find in Wikipedia.  If that solution isn't good enough (or is too much work), then you may be better off spending the money on the official product.


text:
What are the eyesight requirements for LPPL/LAPL in the UK?
I am a UK resident, and I am interested in learning to fly. Unfortunately my eyesight is not great, and I don't believe I meet the requirements for a PPL.
From my research, it seems the medical requirements of the LPPL and LAPL are a lot lower, and I may qualify for these, but I'm a little unsure of the requirements in terms of eye sight requirements.
I currently wear contact lenses, with corrective strength of -9.5 and -10.
Does anyone know what the requirements are under LPPL/LAPL in order to qualify?

I don't know what the requirements are, and certainly not in the UK, which is why I'm not posting this as an answer, but a medical examination by an aviation medical examiner (or equivalent in the UK) isn't that expensive. You could consider just giving it a try and seeing (I know, I know...) how it goes. A UK AME might even be able to answer this over the phone. Either way, it's a good bet that you'll need to have your prescription handy.

In the US we have to meet with an Aviation Medical Examiner (AME) for a very basic medical to get a student pilot license. Does the UK have the same? Ask around at the airport where you're thinking of taking flying lessons.

The CAA has a publication called Light aircraft pilot’s licence (LAPL) Medical certificate - A quick guide for Pilots. It says:

Your GP

Cannot perform the initial assessment if you have one or more of the pre-existing diagnoses that require additional assessment. See
  below:

Decreased visual acuity in either eye below 6/9 despite any correction

In other words, if your corrected eyesight is worse than 6/9 (whatever that means) you need to go to an AME for the medical, otherwise the GP can issue it.
Unfortunately the guide is unclear (as far as I can see) on what exactly the AME will do:

If your GP decides he/she cannot
  issue a LAPL medical certificate
  you will need to see an AME for
  assessment.

I don't know if that means the AME can issue an LAPL medical with limitations, or require you to get a class 2, or something else. Unless someone else can find more information, it may be easiest to call an AME and ask them.

To add to this, numerous pilots hold a medical that has limitations. I can't remember the exact wording on the medical but it's something like "Must wear corrective lenses and must carry a spare pair of corrective lenses" - I usually wear contact lenses and keep a pair of spectacles in my bag.

Afaik, except of Class 1 (not required for PPL nor LAPL) there is no sharp limit for corrective strength. The only requirement is gaining 100% sharpness at both eyes combined, 70% at single eye. Anyway, I agree with  Michael Kjörling that you should contact an UK AME and discuss your exact prescription first, strength is not the only parameter.

This doesn't seem to answer the question about LAPL specifically


text:
Is recurrent simulator training required every 12 months whilst acting as SIC under Part 91?
What are the regulations on recurrent training for a crew member who is type rated as PIC but is acting as SIC, flying internationally on a Bombardier Global Express Part 91 operation (not subpart K)? The last recurrent training for the crew member was 18 months ago.
Part 61.55 stipulates SIC qualifications but i would like to know if recurrent simulator training is required every 12 months whilst acting as SIC? If not, then what 'currency' values are required to maintain your privileges as SIC?

Welcome to aviation.SE! Is this a Part 91 Subpart K operation?

No not a sub part k operation

The only Part 91 recurrent training and testing requirements are in subpart K, so if they don't apply to you then only the general currency requirements in Part 61 would apply.
The SIC currency requirements in 61.55 are (every 12 months):

(1) Become familiar with the following information for the specific
  type aircraft for which second-in-command privileges are requested—
(i) Operational procedures applicable to the powerplant, equipment,
  and systems.
(ii) Performance specifications and limitations.
(iii) Normal, abnormal, and emergency operating procedures.
(iv) Flight manual.
(v) Placards and markings.
(2) Except as provided in paragraph (g) of this section, performed and
  logged pilot time in the type of aircraft or in a flight simulator
  that represents the type of aircraft for which second-in-command
  privileges are requested, which includes—
(i) Three takeoffs and three landings to a full stop as the sole
  manipulator of the flight controls;
(ii) Engine-out procedures and maneuvering with an engine out while
  executing the duties of pilot in command; and
(iii) Crew resource management training.

Practically speaking, what the FAA requires is only part of the picture because your insurance company will probably mandate certain training and currency requirements that may go well beyond the regulations.


text:
What is the usual range of Approach and En Route ATC VHF radios?
I was wondering what is the regular range of Approach VHF radios used by ATC (normally 50W)? And what about En Route VHF radios (usually 100 or 250W)?

Are you asking about frequency ranges or the distance at which the radios can effectively be received. The first is a hard range the second will depend on altitude and terrain.

VHF radios are line-of-sight, and typically the range is limited not by the transmit power but by terrain and the aircraft's horizon. PPL students are taught a rule of thumb: the range in nautical miles is the square root of 1.5 times the aircraft's altitude in feet. This makes your range roughly 40 nm at 1000 ft, 90 nm at 5000 ft, and 120 nm at 10000 ft.
En-route stations intended to be used over a wide area will typically use several transmitters on high ground many miles apart, to ensure reception over their whole operating area. Even so, low-level flying can put you into a blind spot between transmitters.

I have not found the Approach radios to be receivable at that range. That would imply Boston (MA) Approach would be receivable to nearly Albany, NY. I know that at 6,500ft they are not. Their service areas are also much smaller. Going westward, Boston hands off to NY Center, then Bradley Approach, then Albany Approach, unless you climb higher to stay in NY Center's domain for longer.  Flying down to the Cape, 91nm, an even shorter distance (about 10 miles), one changes frequency several times as well, and the main Boston radio cannot be picked up that far away. No terrain in between either.


text:
Why did my flight descend for landing then suddenly climb steeply and turn sharply?
I was flying on a Vueling flight (VLG6251) from Rome, Italy to Orly, France on the 31st of December 2017. During landing, my plane went to a really low altitude as if it was about to land but there was no airport! I mean it was really low, and all of the sudden the plane sped up as if was taking off and it went to really high altitude really fast, while sharply turning to the left! Then I saw the airport. I checked the flight history of that flight and there was no history of it doing that.
Any suggestions on what was happening?

Just sounds like a go-around. When you say you checked the flight history where did you look? A radar track would pretty much tell if that's the case, but flights that far back are paywalled

Also bear in mind that the back of a plane is the worst possible place to see an airport. If it was an approach where the plane flew towards the runway from miles out then you would never see it

According to data recorded on Flightradar24 and the Planefinder App, that particular flight did not go around; however the turn to final approach was flown at lower altitude than some other flights that day (this seems different but not necessarily uncommon or abnormal, and happened for unknown reasons that cannot be inferred from the data).
Screenshot from the app:


text:
How are compressor blades connected to the shaft in turbojet engines?
The below figure is about rotor and stator blades of compressor . How these are connected to the compressor shaft in jet engines ?

Welcome to Aviation.SE! Could you edit your question to ask only one specific question per post? If you have multiple question you should ask them separately. It would be a good idea if you took our site [tour] and checked the [help] to see how the site works.

You’re asking about rotor blades. Why not specify the compressor rotor? People may think you’re enquiring after a helicopter rotor.

I would suggest reversing the terms to read "how are compressor blades connected to the rotor?"  I think that is more correct, and might clear up some some confusion.

There are three primary ways that blades are connected to the shaft

Circumferential dovetails  Here is a nice site demonstrating it.  Includes STL files so you can 3D print your own demonstration part.
https://cults3d.com/en/3d-model/tool/jet-engine-component-1-1-axial-compressor-circumferential-dovetail-slot-type
Axial dovetails Here is a video demonstrating blades with an axial dovetail being assembled.  This particular video is a turbine, not a compressor, but in theory the same method can be used.

Blisk "Blisk" is a contraction of "bladed disk". The blades and the disk are physically welded together (or in some cases machined as one piece out of a solid block of metal).  Here's a wikipedia article on blisks
https://en.wikipedia.org/wiki/Blisk

Each of these methods has advantages and disadvantages in terms of weight, cost, vibration, etc.  Which one is picked depends on the circumstances.  It is also common to see multiple different methods within one compressor. For example the LEAP-1A HPC has a 10 stage compressor.  The forward stages are blisks, and the aft stages are circumferntial dovetails.